Photomultiplier having a photocathode comprised of a compound semiconductor material

Abstract

A photoelectric emission surface which is excellent in stability and reproducibility of photoelectric conversion characteristics and has a structure capable of obtaining a high photosensitivity is provided. A predetermined voltage is applied between an upper surface electrode and a lower surface electrode by a battery. Upon application of this voltage, a p-n junction formed between a contact layer and an electron emission layer is reversely biased. A depletion layer extends from the p-n junction into the photoelectric emission surface, and an electric field is formed in the electron emission layer and a light absorbing layer in a direction for accelerating photoelectrons. When incident light is absorbed in the light absorbing layer to excite photoelectrons, the photoelectrons are accelerated by the electric field toward the emission surface. The photoelectrons obtain an energy upon this electric field acceleration, and are transitioned, in the electron emission layer, to a conduction band at a higher energy level, and emitted into a vacuum.

What is claimed is: 
     
       1. A photomultiplier having a photocathode for emitting electrons in correspondence with light incident on said photocathode, comprising: a substrate of p-type InP, which has a carrier concentration of not less than 1×10 18  cm -3  ;   a first layer of p-type InGaAsP, which has a carrier concentration of not more than 5×10 16  cm -3 , being in contact with said substrate;   a second layer of p-type InP, which has a carrier concentration of not more than 5×10 16  cm -3 , being in contact with said first layer, and having an exposed surface for emitting photoelectrons;   a third layer of n-type InP, which has a carrier concentration of not less than 1×10 18  cm -3 , being in contact with said second layer;   an upper surface electrode defining an opening suitable to permit emission of photoelectrons therethrough, being in contact with said third layer;   an active layer for decreasing a work function of said second layer in contact with said exposed surface of said second layer; and   a lower surface electrode being in contact with said substrate.   
     
     
       2. A photomultiplier according to claim 1, wherein said active layer is comprised of Cs. 
     
     
       3. A photomultiplier according to claim 1, wherein said active layer is made of a material selected from the group consisting of CsO and CsF. 
     
     
       4. A photomultiplier according to claim 1, further comprising: a sealed vessel which accommodates said photocathode;   a first stage dynode arranged in said sealed vessel;   a focusing electrode arranged between said first stage dynode and said photocathode;   a plurality of dynodes, including an ultimate stage dynode and arranged contiguously from said first stage dynode;   an anode arranged near said ultimate stage dynode.   
     
     
       5. A photomultiplier according to claim 1, wherein a potential higher than that of said lower surface electrode is applied to said upper surface electrode. 
     
     
       6. A photomultiplier according to claim 4, further comprising an internal conductive film coated on an inner wall of said sealed vessel surrounding a space reserved between said focusing electrode and said photocathode and electrically connected to said upper surface electrode. 
     
     
       7. A photomultiplier according to claim 1, further comprising: a sealed vessel which accommodates said photocathode, having a predetermined portion through which light is input to said photocathode; and   a transparent electrode coated on an inner surface of said predetermined portion of said sealed vessel and electrically connected to said lower surface electrode.   
     
     
       8. A photomultiplier according to claim 1, wherein said third layer has pattern shape for uniformly distributing and exposing said second layer. 
     
     
       9. A photomultiplier having a photocathode for emitting electrons in correspondence with light incident on said photocathode, comprising: a substrate of a first conductivity type semiconductor, having a predetermined carrier concentration;   a first layer of a first conductivity type semiconductor, having a first carrier concentration, being in contact with said substrate;   a second layer of a first conductivity type semiconductor, having a second carrier concentration, being in contact with said first layer, and having an exposed surface for emitting photoelectrons;   a third layer of a second conductivity type semiconductor, having a third carrier concentration, being in contact with said second layer;   an upper surface electrode being in contact with said third layer;   an active layer for decreasing a work function of said second layer in contact with said exposed surface of said second layer; and   a lower surface electrode being in contact with said substrate.   
     
     
       10. A photomultiplier according to claim 9, wherein a first energy bandgap of said substrate is larger than a second energy bandgap of said first layer, and a third energy bandgap of said second layer is larger than said second energy bandgap. 
     
     
       11. A photomultiplier according to claim 9, wherein a fourth energy bandgap of said third layer is substantially equal to said third energy bandgap of said second layer. 
     
     
       12. A photomultiplier according to claim 9, wherein said third concentration is higher than 1×10 18  cm -3 . 
     
     
       13. A photomultiplier according to claim 9, wherein said substrate is p-type InP, said predetermined carrier concentration is not less than 1×10 18  cm -3 ,   said first layer is p-type InGaAsP,   said first carrier concentration is not more than 5×10 16  cm -3 ,   said second layer is p-type InP,   said second carrier concentration is not more than 5×10 16  cm -3 ,   said third layer is n-type InP, and   said third carrier concentration is not less than 1×10 18  cm -3 .   
     
     
       14. A photomultiplier according to claim 9 wherein said active layer is comprised of Cs. 
     
     
       15. A photomultiplier according to claim 9, wherein said active layer is comprised of a material selected from the group consisting of CsO and CsF. 
     
     
       16. A photomultiplier according to claim 9, further comprising: a sealed vessel for accommodating said photocathode;   a first stage dynode arranged in said sealed vessel;   a focusing electrode arranged between said first stage dynode and said photocathode;   a plurality of dynodes including an ultimate stage dynode and arranged contiguously from said first stage dynode; and   an anode arranged near said ultimate stage dynode.   
     
     
       17. A photomultiplier according to claim 9, wherein said third layer has pattern shape for uniformly distributing and exposing said second layer. 
     
     
       18. A photocathode comprising: a substrate of a first conductivity type semiconductor having a predetermined carrier concentration;   a first layer of a first conductivity type semiconductor, having a first carrier concentration, being in contact with said substrate;   a second layer of a first conductivity type semiconductor, having a second carrier concentration, being in contact with said first layer, and having an exposed surface for emitting photoelectrons;   a third layer consisting of a semiconductor of a second conductivity type, having a third carrier concentration, being in contact with said second layer;   an upper surface electrode being in contact with said third layer;   an active layer for decreasing a work function of said second in contact with said exposed surface of said second layer; and   a lower surface electrode being in contact with said substrate.   
     
     
       19. A photocathode according to claim 18, wherein a first energy bandgap of said substrate is larger than a second energy bandgap of said first layer, and a third energy bandgap of said second layer is larger than said second energy bandgap of said first layer. 
     
     
       20. A photocathode according to claim 18, wherein a fourth energy bandgap of said third layer is substantially equal to said third energy bandgap of said second layer. 
     
     
       21. A photocathode according to claim 18 wherein the third concentration is higher than 1×10 18  cm -3 . 
     
     
       22. A photocathode according to claim 18, wherein said substrate is p-type InP, said predetermined carrier concentration is not less than 1×10 18  cm -3 ,   said first layer is p-type InGaAsP,   said first carrier concentration is not more than 5 ×10 16  cm -3 ,   said second layer is p-type InP,   said second carrier concentration is not more than 5×10 16  cm -3 ,   said third layer is n-type InP, and   said third carrier concentration is not less than 1×10 18  cm -3 .   
     
     
       23. A photocathode according to claim 18 wherein said active layer is Cs. 
     
     
       24. A photocathode according to claim 18 wherein said active layer is comprised of a material selected from the group consisting of CsO and CsF. 
     
     
       25. A photomultiplier according to claim 18, wherein said third layer has pattern shape for uniformly distributing and exposing said second layer.